This invention relates generally to the material handling art, and more particularly to a method of and apparatus for lifting a load utilizing an existing bridge or gantry-type crane in combination with a portable lifting beam or Strongback adapted to be lifted into operable position and partially supported by the bridge crane.
This invention finds particular usefulness in aluminum reduction plants for lifting and transporting spent pot shells used in the electrolytic production of aluminum. In the commercial production of aluminum which is carried out by the Hall-Heroult process, aluminum oxide, refined from bauxite ores, is reduced electrolytically in an aluminum reduction cell, usually referred to as a pot. The pot includes an upper assembly through which one or more carbon anodes extend downwardly into the interior thereof, and a carbon-lined steel lower shell which contains the molten metal and which serves as the cathode of the cell. In the process, alumina, the solute is dissolved in molten cryolite, the solvent, at a temperature of approximately 970.degree.C. The dissolved alumina, when subjected to an external current source, disassociates into positive aluminum and negative oxygen ions. As aluminum is produced, it is periodically syphoned off from the pot.
A typical aluminum reduction plant includes several hundred pots divided into a plurality of potlines, each of which might have approximately 100 pots connected in series. Each potline is enclosed in a room or bay extending several hundred feet in length. Each potline is provided with a bridge or gantry-type crane for servicing the various pots contained therein.
During operation of the cathodic lining of the lower shell absorbs bath material causing it to expand and distort the shell, and almost doubling its weight. The lining normally deteriorates gradually and has a useful life of from 1 to 5 years (1200 days average). When the shell becomes "spent", the old carbon lining must be removed and the shell then relined with new material. However, because of the aforementioned buildup of weight in a spent shell, the shell will be too heavy to be lifted by the existing bridge or gantry crane. Consequently, in prior art systems the shell would have to be relined in place and the pot station would have to be out of service for the length of time required to demolish and remove the old carbon cathode lining, thereafter remove the shell from the line and replace it with a spare shell, and then reline the new shell in place. During this time, of course, the pot station was out of operation, and plant capacity was reduced accordingly. In a plant containing several hundred pots, there would normally be at least two or three pots down at any given time. Moreover, maintenance of a pot in line involved risk to personnel working under the existing hazardous conditions.
Of course, if the overhead bridge or gantry crane had the capacity to lift a spent shell prior to removal of the bath-absorbed carbon lining therefrom, this downtime and risk could be substantially reduced. However, because large aluminum reduction plants include several potlines or bays, each serviced by a separate bridge crane, it would involve considerable capital expenditure to upgrade each of these bridge cranes to the point where they would have the capacity to lift a spent pot shell which could weigh approximately 95 tons. Moreover, the full capacity of such an upgraded crane would be utilized only a small fraction of the time, and the normal daily operations, such as carrying tapping crucibles and other equipment, would be uneconomically and cumbersomely performed by a crane having a much greater capacity than necessary.